There are very many known techniques of image compression to reduce the quantity of data needed to represent an image or a sequence of moving images. Thus, it is sought especially to reduce the bit rates of the digital signals in order to transmit them and/or to store them on a data carrier.
The invention can be applied especially but not exclusively to the transmission of image signals at low bit rates as well as to transmission without bit rate guarantee, as in the case of transmission made according to the IP (“Internet Protocol”).
Among the many known image-encoding methods, it is possible to distinguish especially the ISO-JPEG and ISO-MPEG techniques which have given rise to a standard. These encoding techniques rely especially on the implementation of transformations, enabling the efficient elimination of redundancy in an image.
The main transformations used in image compression are:                the Karhunen Loéve transformation (KLT),        the discrete Fourier transformation (DFT),        the discrete cosine transformation (DCT),        and the Walsh-Hadamard transformation (WHT),        
To overcome these various drawbacks, another method has been proposed in the patent document FR-98 12525 filed on behalf of the same Applicant as in the present patent application, entitled “Procédé de codage d'images fixes ou animées avec réduction et adaptation du debit” “Method for the encoding of still or moving images with reduction and matching of the bit rate”.
The object of this technique is a method for encoding a digital image designed to produce a bit stream representing this image, the length of the bit stream being a function of the desired representation. This method comprises the following steps:                the defining, in a field of the image to be encoded, of a hierarchical meshing comprising a plurality of nested meshings for which the peaks of the mesh units may be pixels of said image;        the achieving of optimization of luminance, chrominance, and positions on each meshing level;        the determining, for each mesh unit of said hierarchical meshing, of a difference in luminance between the image to be encoded and an interpolated image obtained from the peaks of the nested meshing to which the mesh unit considered belongs; and        the introducing, into the bit stream, of the values (advantageously encoded differentially with respect to the previous hierarchical level) of positions, luminance and chrominance of the peaks of the mesh units whose luminance difference is greater than a threshold difference.        
It will be noted that this technique is not limited to the luminance and chrominance signals but can be applied to any model of colors. According to this technique, and as illustrated in FIG. 1, at the end of the meshing step, a quaternary tree structure 11 is built. This structure is associated with the hierarchical meshing 12 to manipulate the values (colors and positions) of the peaks of the mesh units. The tree has a number of nodes equal to the number of triangles in the corresponding meshing level. Each node 13 of the tree pertains to a single triangle 14 of the hierarchical meshing 12.
Once this tree 11 is built, a selection is made of the data of the tree to be introduced into the bit stream representing the image that will be transmitted and/or stored. This selection depends on the desired quantity.
To make this selection, for each triangle a difference in luminance between the image to be encoded and the interpolated image is calculated on the basis of the peaks of the nested meshing to which the considered mesh unit belongs. This difference is then compared with a threshold difference for each triangle. The value of the threshold difference is a function of the desired quality of representation.
Then, the part of the tree relating to the triangles whose difference in luminance is greater than the threshold difference is introduced into the bit stream. This process of selecting the nodes of the tree by taking an in-depth route is illustrated in FIG. 2. Only the nodes that are above the borderline 21 are preserved.
The threshold difference therefore enables the transmission of the data relating to the image as a function of the local quality of these different triangular partitions. Indeed, on a textured part, the transmission of the data takes place up to the last meshing level (the finest meshing) and, for the smoother parts, a rough level proves to be sufficient.
This technique is also described by Wang et al in the document “USE OF TWO-DIMENSIONAL DEFORMABLE MESH STRUCTURES FOR VIDEO CODING (XP000641037). This document proposes a method for the generation of a quadrangular hierarchical meshing. An error criterion is used to decide whether or not it is necessary to continue the hierarchical division for a given mesh unit.
To obtain adequate quality, when a portion of the image is highly textured, it is necessary to plan for a large number of levels. In this case, the efficiency of the hierarchical encoding is low. In other words, the hierarchical encoding is well suited to relatively simple images but not to the images having highly textured parts.
In practice, it is not rare for at least a part of the image to be highly textured. This aspect truly limits the practical value of the hierarchical encoding as described here above.
In particular, it is a goal of the invention to overcome these different drawbacks of the prior art.
More specifically, it is a goal of the invention to provide an image encoding method that makes use of the advantages of the technique herein known as hierarchical encoding, without showing any of its drawbacks.
In particular, it is a goal of the invention to provide an encoding method of this kind that is efficient even when the image to be processed has highly textured portions.
Another goal of the invention, naturally, is that the modifications made in the hierarchical encoding technique as described here above should not modify its efficiency when the image or image portion to be processed has low texturing.
It is also a goal of the invention to provide a method of this kind that is simple to implement and especially does not need any manual intervention (for example for the selection of the textured zones).
More generally, a goal of the invention is to provide an image encoding method with a good radio between the quantity of data to be transmitted and/or stored and quality of image reconstruction.
Another goal of the invention is to provide a corresponding decoding method that enables the reconstruction of images simply and at a low cost (in terms of processing time, storage capacity, etc.).
It is also a goal of the invention to provide a decoding method of this kind with high qualities of scalability (i.e. allowing a rough image to be swiftly reconstructed and then gradually refined up to the desired level of refinement).